In the field of electro-acoustic products, the microphone is a sensor that converts acoustic energy into electrical energy. A capacitive micro-electro-mechanical system (MEMS) microphone generally includes a vibration membrane, a fixed plate (back plate), and a cavity between the vibration membrane and the fixed plate. A capacitive MEMS microphone enables detection of a capacitive value change due to the displacement of the vibration membrane caused by a change in the acoustic pressure, and the detected capacitive value change is then converted to an electrical signal for processing.
Comparing to conventional microphones, MEMS microphones are less sensitive to temperature, vibration, humidity and have better mechanical stability. For example, MEMS microphones can withstand high reflow temperature of about 260° C., and the MEMS microphones do not have obvious change in performance.
With continuous development in technology and the growing demand, MEMS microphones are not only expected to provide better acoustic performance, but also to provide better mechanical stability in order to be able to resist deformation during transport and in post-working without damage that may be caused by cracks.
In order to ensure the mechanical reliability and stability of a microphone, a microphone generally needs to satisfy the air pressure test (APT) and the drop test. The air pressure test is performed after the microphone has been packaged. Air is introduced to the microphone from the top or from the bottom of the microphone to test the maximum pressure that the microphone can withstand. Air introduced from the top side of the microphone to test the air pressure is called top-side air pressure test, and air introduced from the bottom side of the microphone to test the air pressure is called bottom-side air pressure test. Drop test is referred to the percentage of microphones that survive without suffering damage after being dropped from a certain height (e.g., 1 meter) onto a floor made of a certain material (e.g., a marble floor).
FIG. 1 shows a structure of a lower electrode plate (vibration membrane) 101 of a MEMS microphone. Lower electrode plate 101 has a middle square portion 111 that can vibrate and is mounted to a semiconductor structure by four anchors 121. The semiconductor structure has an opening in the backside of a substrate that operates as an acoustic channel for collecting sound. In current designs, the size of the opening is substantially equal to the size of middle square portion 111 of the lower electrode plate.